How can I pay Zakat for past years, taking into account my ignorance of the amount of my wealth during those periods, and my current financial inability to pay it?

The Zakat payer must strive to ascertain the amount of Zakat due from him, then disburse it whenever he is able, because Zakat is a debt firmly established in one's liability, and it does not lapse due to insolvency after it becomes due and there is negligence in its payment.